1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus, a photoelectric conversion system, and a method of driving the same and, more particularly, to a photoelectric conversion apparatus and photoelectric conversion system having a p-n junction or a MOS transistor, and a method of driving the same.
2. Description of the Related Art
In, for example, a p-n junction of a sensor where photoelectric conversion occurs, the number of crystal defects grows due to X-rays and cosmic rays in the nature or X-rays generated from medical equipment using X-rays.
Since the defect amount increases along with the passage of time, long-term use of a sensor makes its performance unsustainable.
A conventional method of preventing such degradation keeps a semiconductor at a predetermined temperature, as described in Japanese Patent Laid-Open No. 2002-114276.
However, in the technique described in Japanese Patent Laid-Open No. 2002-114276, the anti-degradation effect is only provided when using a metal material such as lead. It is impossible to place such a film on the light-receiving surface of a photosensor. It is also impractical to keep a sensor at a predetermined temperature.
Generally, a MOS semiconductor element irradiated with radiation generates electron-hole pairs in its gate oxide film. The interface between the silicon and the silicon oxide film traps and accumulates the holes moving to there.
This causes impediments such as fluctuations of the threshold voltage, as is known.
A technique disclosed in Japanese Patent Laid-Open No. 57-162358 sets the power supply line or signal line to the ground potential when the element is not used under radiation irradiation so as to produce an annealing effect and release charges generated by radiation.
However, in the technique disclosed in Japanese Patent Laid-Open No. 57-162358, if the signal line connected to a photoelectric conversion element is set to the ground potential, and a photoelectric conversion operation is performed after potential application, electrons generated by the photoelectric conversion element make the potential of the signal line negative. Since the output circuit connected to the signal line cannot output a signal, image capturing is impossible.
A technique disclosed in Japanese Patent Laid-Open No. 2001-111020 fixes the terminals of a pixel except a photoelectric conversion element to a predetermined potential during at least a time determined by the area occupied by a driving circuit and the photoelectric conversion element.
However, in the technique disclosed in Japanese Patent Laid-Open No. 2001-111020, it is impossible to fix the potential of the terminals after the image sensing apparatus is powered off. While the image sensing apparatus is not used, degradation caused by X-rays is nonsuppressible. Additionally, since the photoelectric conversion element is excluded from the fixture, the electric field applied to it is not relaxed so no effect of preventing degradation by X-rays is expected.
A technique described in Japanese Patent Laid-Open No. 2005-175526 applies a refresh potential different from a normal reset potential to one terminal of a photoelectric conversion element to sweep holes staying in an intrinsic semiconductor layer included in the photoelectric conversion element.
However, in the technique described in Japanese Patent Laid-Open No. 2005-175526, the refresh potential applies a negative potential to the remaining terminals of the photoelectric conversion element and also applies an electric field having a magnitude of not 0 to the photoelectric conversion element. This technique cannot therefore prevent degradation by X-rays, either.
Japanese Patent Laid-Open No. 11-17160 discloses the following technique. When all terminals of a solid-state image sensing apparatus are open, the electric field applied to a transfer electrode between a pixel unit and a CCD and an oxide film under it is changed to zero. This suppresses a threshold value shift or an increase in the level of the interface between a semiconductor and an oxide film due to ion damage even when radiation irradiates the pixel unit of the CCD.
However, this technique does not assume anti-degradation under radiation irradiation in the pixel circuit arrangement of an amplification-type photoelectric conversion apparatus having, for example, an amplification MOS transistor in a pixel.
In a photoelectric conversion apparatus using a photoelectric conversion element, an increase in the leakage current greatly affects the quality of output signal. It is therefore necessary to minimize current leakage.
If a photoelectric conversion element includes a p-n junction, defects generated by X-rays or cosmic rays depend on the magnitude of the electric field applied to the p-n junction, as is apparent from experiments.
A normal photoelectric conversion apparatus is designed to stably operate using an external or internal parasitic capacitance when a voltage to be applied to the circuit is stably supplied. Most photoelectric conversion apparatuses have a large external capacitance of several ten μF connected between terminals such as the power supply terminal and GND terminal.
On the other hand, most analog circuits for processing a signal based on signal charges generated by a photoelectric conversion element are biased by a constant current. Hence, a predetermined current is supplied to the processing circuit independently of the magnitude of the power supply voltage.
As a characteristic feature, the constant current circuit stops its operation when the power supply voltage drops to a predetermined value or less, and after the stop, the resistance between the power supply and the GND becomes very high.
Hence, during an inoperative state of the photoelectric conversion apparatus, the power supply terminal voltage decreases to a predetermined value because of discharge caused by current consumption. At the voltage that stops the operation of the constant current circuit or less, the voltage tends to be maintained.
The photoelectric conversion unit basically must be used in a high impedance state. For this reason, after the terminating the operation of the photoelectric conversion apparatus, charges by, for example, the parasitic capacitance sometimes keep an electric field applied to the p-n junction of the photoelectric conversion unit for a very long time. This may cause degradation of the junction by X-rays.
The gate potential of a MOS transistor included in a pixel is rarely controlled to change to the ground potential every time the power is turned off. In this case, an electric field is kept applied between the gate and drain and between the gate and source of the MOS transistor even after power-off. Consequently, the electric field applied between the gate and source (or drain) of the MOS transistor generates a hot carrier, which is trapped by the gate oxide film and subsequently cause fixed noise signal or generates a substrate noise current.